What is the Best gun for black bear?

That's a good question, depends on the size of the person using the gun

I'd recommend a large caliber revolver, 44 magnum and up, so that it never jams in a time of need, with hollowpoint rounds to maximize size of wound. If your are worried about bear attack make sure the revolver fits your hand so you can have a good, controlling grip on the revolver and that you can practice and can shoot accurately with it without flinching from the recoil.